1. Field of the Invention
The present invention relates to an information recording medium such as an optical disk, and more particularly to a direct-read-after-write optical disk using an organic dye film for a recording film.
2. Description of the Related Art
As optical disks as information recording mediums, there are a reproduction-only optical disk as typified by a CD or a DVD-ROM, a write once read many optical disk as typified by a CD-R or a DVD-R, a rewritable optical disk as typified by a CD-RW, a DVD-RAM or a DVD-RW which can be utilized in an external memory for a computer or a recording/reproducing video machine, and others.
Of the above-described optical disks conforming to various standards, in the write-once read-many optical disk, a groove (guide groove) formed on the optical disk is wobbled and address information is provided to the groove.
Many of the direct-read-after-write optical disks take a structure in which an organic dye film with a predetermined thickness is deposited in a groove previously formed when molding the disk.
On the other hand, the direct-read-after-write optical disk must include a prepit (groove in which physical information is recorded) such as a lead-in groove in order to maintain compatibility with a reproduction-only optical disk.
However, since an organic dye material as a recording film is applied on the prepit previously formed on a substrate, it is known that a depth of the prepit may not be necessarily even.
On the basis of such a background, in Jpn. Pat. Appln. KOKAI Publication No. 2003-203391, the following expression is specified:λ/4≦(n1−n2)d1+n2·d2≦λ/2wherein n1 is a refractive index of a disk substrate,                n2 is a refractive index of a dye,        λ is a wavelength of light beams,        d1 is a depth of a prepit (groove) on the disk substrate, and        d2 is a depth of the prepit (groove) on the dye.        
However, in the direct-read-after-write optical disk which is used based on the standard in which a wavelength of laser beams utilized for recording is reduced to approximately 400 nm (reduction in diameter of a condensing spot diameter) for the purpose of increasing a recoding density, even if depths of the pit and the groove portion are to be specified by using the condition described in the patent cited reference 1, since a track pitch (distance between grooves) is narrower than a spot diameter of light beams used for recording and reproduction of information, there is a problem that an optimum value achieving both a signal amplitude of the pit portion and the tracking stability of the groove portion cannot be obtained.
It is to be noted that, in an optical disk utilizing a wavelength of laser beams for recording of approximately 400 nm, the extent to which the pit is filled with an organic dye material used for a recording film is decided by the width of the pit, and, as the pit depth fluctuates, the quantity of the dye material which enters the pit must be uniformized.